Anisotropy In Hydrogenated Silicon Thin Films
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Anisotropy In Hydrogenated Silicon Thin Films John D. Webb, B. P. Nelson, A. H. Mahan, J. Theisen, R. Reedy, J. D. Perkins and L. M. Gedvilas National Renewable Energy Laboratory, Golden, Colorado, USA Abstract We investigated spatial non-uniformity and optical anisotropy in hydrogenated silicon thin films using infrared spectroscopy and other techniques. The films ranged in morphology from amorphous to micro-crystalline, as determined by Raman spectroscopy and X-ray diffraction (XRD). Trace element analysis of the films was carried out using secondary ion mass spectrometry (SIMS). We used polarized attenuated total reflection (ATR) measurements together with Fourier transform infrared (FTIR) spectroscopy to investigate anisotropy in the Si-H covalent bonding structure of the films, and also to detect oxide impurity phases present at certain sites in some samples. The FTIR-ATR measurements were performed using a germanium (Ge) contact microprobe with a 100-micron sampling area, as well as on films deposited on standard crystalline Si and Ge ATR substrates. The Ge contact ATR microprobe enabled relatively interference-free measurements of the IR spectra of a-Si:H and µc-Si:H films deposited on conductive substrates such as stainless steel and transparent conductive oxides, and is also sensitive to the spatial distribution of oxide precipitates in the films. This measurement is difficult or impossible using transmission or reflection IR spectroscopy. Introduction In earlier work [1], we reported applications of polarized attenuated total reflection (ATR) spectroscopy to the analysis of hydrogenated amorphous silicon films. In particular, we reported a highly-polarized infrared band with a maximum at 1220-1230 cm-1 and correlated the intensity of this band with the concentrations of oxygen and carbon impurities in the films. We also observed that the intensity of the Si-H wagging band at 640 cm-1 is lower in p-polarization than in S-polarization for a-Si:H films on Si ATR substrates, but not for a-Si:H films on CdTe ATR substrates. We reported that the band at 1220-1230 cm-1 is not present in a-Si:H films on CdTe ATR substrates. The polarization-dependent features of the ATR spectra were not reported in earlier work [2]. In view of the carrier transport anisotropy observed in micro-crystalline silicon[3], we investigated the optical anisotropy of a series of films ranging from a-Si:H to µc-Si:H in morphology. Experimental Films of amorphous to micro-crystalline silicon ~ 1 µm in thickness were deposited on 1 x 5 x 5 mm or Si 45 degree ATR substrates or on 1 x 5 x 10 mm sixty degree Ge substrates. Depositions were via HWCVD or PECVD, with extent of microcrystallinity controlled by hydrogen dilution [4]. Substrates were cleaned using solvent rinses or by HF
A16.4.1
immersion. Films ~1 micron thick, ranging in structure from amorphous to micro-crystalline silicon were deposited onto stainless steel or tin oxide-coated glass substrates. The substrates were cleaned using aqueous detergents followed by solvent rinses and
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